The present invention relates to a method of machining silicon nitride ceramics and silicon nitride ceramic products, specifically sliding parts which are brought into frictional contact with metal parts at high speed, such as adjusting shims, rocker arms, roller rockers, cams, piston rings, piston pins and apex seals, and bearing parts such as slide bearings and roller bearings.
Silicon nitride ceramics are known to have excellent mechanical properties in hardness, strength, heat resistance, etc. and possess a big potential as materials for mechanical structures. But silicon nitride ceramics are typically hard but brittle materials. Therefore, it is required to select an appropriate machining method for providing a geometric shape as required by the end products and also to improve the strength and durability of the finished products.
At the present time, the best-used method for machining silicon nitride ceramics is grinding with a diamond grinding wheel. But this method tends to leave damage such as cracks on the machined surface, which will lower the strength and reliability. This has been a major obstacle to the application of these materials.
For example, as Ito points out (in a book titled "Recent Fine Ceramics Techniques", page 219, published by Kogyo Chosakai in 1983), there is a correlation between the surface roughness of silicon nitride ceramics machined by grinding and the bending strength and it is required to keep the surface roughness below 1 micrometer to ensure reliability in strength. Also, as has been pointed out by Yoshikawa (FC report, vol 8, No. 5, page 148, 1990), the depth of cracks formed when grinding depends on the grain size of the diamond grinding wheel used. Such cracks formed in silicon nitride ceramics materials may be as deep as 20-40 micrometers (or microns). Cracks of this order can make the end product totally useless.
As shown in Japanese Patent Unexamined Publication 63-158070, silicon nitride ceramics having a bending resistance of 100 kg/mm.sup.2 or more under JIS R1601 are especially difficult to grind with an ordinary diamond grinding wheel. Also, the possibility of causing surface damage increases.
It is known to finish a surface damaged by normal grinding with a diamond grinding wheel by polishing or lapping with abrasive grains to remove any damaged surface and thus to increase the strength of the product. But such a method is extremely problematic from an economical viewpoint.
But the grinding method using a diamond grinding wheel is superior in flexibility of machining facility and machining cost. Thus, it is essential to establish a method of grinding silicon nitride ceramics with a diamond grinding wheel without the fear of surface damage. One way to remove the influence of surface damage was disclosed Kishi et al ("Yogyo Kyokai Shi", vol. 94, first issue, page 189, 1986), in which after grinding .beta.-Sialon, a silicon nitride ceramic, it is subjected to heat treatment at 1200.degree. C. in the atmosphere to form an oxide layer on its surface to fill the damaged parts with the layer and improve the strength. It is known that this method can increase the bending strength, its reliability and the Weibull modulus of the material ("Yogyo Kyokai Shi", vol. 95, sixth issue, page 630, 1987).
But in this method, since the heat treatment is carried out after finishing the material into a final shape, the dimensional accuracy tends to decrease. Also, as pointed out by Kishi et al ("Yogyo Kyokai Shi", vol. sixth issue, page 635, 1987), this method has a problem in that it is difficult to keep down variations, depending upon the size of the damage on the material before heat treatment. Thus, it is difficult to use this method in the actual production.
In order to solve these problems, it is necessary to develop a machining method which provides a sufficiently smooth surface roughness (e.g. Rmax&lt;0.1 micrometer) and by which the surface damage such as cracks can be repaired after grinding or even during grinding.
One method of this type is disclosed by Ichida et al ("Yogyo Kyokai Shi", vol. 94, first issue, page 204, 1986), in which a mirror finish is obtainable by grinding a .beta.-Sialon sintered body with a fine-grained diamond grinding wheel while forming flow type chips. Also, Ito shows that it is possible to form a mirror finish by grinding silicon nitride ceramics with an ordinary alumina grinding wheel ("Latest Fine Ceramics Techniques", published by Kogyo Chosakai, page 219, 1983).
The finished surfaces obtained by these techniques show a maximum height-roughness Rmax of 0.03 micrometer. Considering the fact that the crystal grain diameters of silicon nitride and .beta.-Sialon are both several micrometers, it appears the statements of Ichida and Ito, that is, "removal of material by forming flow type chips chiefly by plastic deformation" and "removal of material mainly by abrasion and microscopic crushing" cannot fully explain the above phenomenon. Further, in the former literature, the work is a pressureless sintered body. It is somewhat inferior in mechanical properties compared with silicon nitride ceramics, which are expected to be widely used for precision machining parts in the future. In this respect, the mechanism of material removal is dependent upon the properties of the material.
It is an object of the present invention to provide an industrially feasible grinding method which can provide a sufficiently smooth finished surface, i.e. a surface having a maximum height-surface roughness Rmax of 0.1 micrometer or less and a ten-point mean roughness Rz of 0.05 micrometer and which can repair any surface damage during grinding.